It's a time saving tool that takes much of the labour out of breaking down your poses - once you are happy with the timing of the main blocking of your animation. And best of all, it's completely free.

What is the tweenMachine?
The tweenMachine is a free Maya plugin designed to simplify the process of creating breakdown poses between your key poses.

Who created it?
The tweenMachine was created by animator Justin Barrett.

How do you install it?
The tweenMachin comes with a Read Me file (which you should read. D'oh!).
To install it, first find these two files: tweenMachine.ziptweenMachine.xpm

1.
Copy the files into your scripts folder in Maya. The path should
look something like this: username/myDocuments/maya/2016/scripts
2. Once you have copied the files into your Maya scripts folder, open Maya.
3.
Now open up the script editor (bottom right hand corner of yr
screen) and type this in the MEL (not Python) command line:
tweenMachine. This should launch tween machine.
4. In the
script editor, copy the command, and create a shelf tab so you can use
the tweenMachine any time you like. Name it tween
5. Click on the shelf tab to activate the tweenMachine. A pop-up slider should appear:

How do you test it out?

To test the tweenMachine, first create some break down poses on a simple moving object, such as a sphere.

1. Create a sphere. Create/polygon primitives/sphere.
2. Move the sphere to one end of the grid, and set a keyframe on it
3. Click on the preferences tab in Maya, and set your animation curves to stepped curves, not spline.
4. Back in your timeline, go to frame 25. In the viewport, move the sphere to the other end of the grid, and set a key frame. Press play. You should now see the sphere move, but in a very staccato fashion.
5. Go to frame 13 in your time line. Set a keyframe on the sphere, and click on the tweenMachine button. Use the slider on the tweenMachine to adjust the position of the breakdown pose. The slider allows you to favour one key pose over another.

Ok, got it. Now, how do I use it for proper animation?
Now use the tweenMachine to create break down poses on a complex human biped.

1. Import a human biped, such as the Morpheus or Max rig.

2. Create two poses, one at frame 1, and a second at frame 25, following the thumbnails below.

3. Now select all the controls, and set a keyframe at frame 13. Click on the tweenMachine tab, and use the slider to adjust the spacing of the breakdown position.

4. Once you have done your pose at frame 13, go ahead and add breakdown poses at frame 5, 9, 17 and 21, using the tweenMachine.

In conclusion:

What the tweenMachine does is allow you to set breakdown positions on all the control curves, without having to manipulate individual controls one by one. It saves a huge amount of time on production, allowing you to concentrate on the important stuff - the performance